With more than Smillion persons infected with HIV by the end of year 2003, urgent measures are required to control HIV epidemic in India. An efficacious vaccine to prevent new HIV infections will be a cost effective tool for control of epidemic. Majority vaccine strategies have relied on the vaccine strains matching the viruses circulating in the target population. This necessitates in depth knowledge of the viruses in circulation and the evolutionary trends. India has initiated vaccine programme and is likely to conduct first vaccine trial soon. Hence generation of molecular epidemiology data will prove a critical input for the programme. This [unreadable]"" study is aimed at generating such data among two major high risk groups, STD clinic attendees in Pune and Chennai and Intra-venous drug users in North Eastern states of Manipur and Nagaland. To study the transmitted viruses, the study will focus on virus sequences from incident infections and inter-clade recombinant strains. Patients attending STD and IVDU clinics will be screened for HIV antibodies. Those positive for antibody will undergo detuned EIA for identifying recent HIV infections. Reverse-transcriptase PCR will be carried out on the plasma sample from each antibody positive individual, HIV specific gag and env sequences will be amplified and HMA analysis will be carried out using kits provided by NIH AIDS Referfence and Reagents programme. The samples displaying different subtype specificity of gag and env amplicons will be considered as recombinant: Full length sequencing will be carried out (as per protocols by Lole et al J.Virol. 1999;73:152) for all samples identified as recombinants and those identified as early HIV infection in detuned EIA. Full length sequences will be analyzed for sequence variations, glycosylation patterns and length variations in env gene. Phylogenetic analysis and recombinat breakpoint analysis will be carried out. The data generated from the study will be first incident estimates in different high prevalence states of India. The molecular virological study will yield data on the evolution of recombinant strains and strains that are presently being transmitted. Northeastern states of India have shown emergence of recombinant strains. Recombinant strains are also reported in other parts of the country. Recombinant hotspots have been reported in neighboring Myanmar and China. This study will help in generating the data that may have implication on future trend of epidemic in India.